This invention relates to apparatus and methods for sampling atmospheric gases and particles.
During the last decade, diffusion denuders have been used in a variety of atmospheric monitoring studies to collect gaseous atmospheric pollutants (Durham et al., 12 Atmospheric Environment 883 (1978); Ferm, 13 Atmospheric Environment 1385 (1979); Shaw et al., 16 Atmospheric Environment 845 (1982); Forest et al., 16 Atmospheric Environment 1473 (1982); Braman et al., 54 Analytical Chemistry 356 (1982)). In these studies, glass or metallic hollow tubes were coated to selectively collect the different gases while allowing other gases and particles to penetrate. These tubular denuders allow high collection efficiencies only for low collection flow rates (less than 1 liter/minute). In 1983, Possanzini et al. developed the annular denuder which efficiently collects atmospheric gas at higher flow rates (higher than 10 liters/minute). Koutrakis et al., 22(1) Atmospheric Environment 157 (1988b) presented an improved method for measuring strong acidity of atmospheric aerosols using an NH.sub.3 diffusion denuder. The denuder was constructed using a foil laminate honeycomb consisting of hexagonal cells of size 0.635 cm, and length 5.08 cm.
Use of a series of two annular denuders (coated with sodium carbonate and citric acid, respectively) makes possible the collection of acidic gases, such as SO.sub.2, HNO.sub.3, HNO.sub.2, gas phase organic acids, and basic gases, such as NH.sub.3. The use of a filter pack downstream from the annular denuders allows the simultaneous collection of particles. This simultaneous collection of particles and gases is feasible only for high flow rates since, for collection flow rates less than 1 liter/minute, the collected particulate matter is not usually enough for analysis unless multiday samples are collected. This is not generally desired for atmospheric chemistry studies where a fine resolution of concentration profiles is necessary to understand processes affecting the origin and fate of atmospheric pollutants. A glass impactor which can be placed upstream from the denuder/filter pack system is useful for removal of coarse particles from the air sample. This inlet is preferably inert, so that losses of reactive gases such as nitric acid, sulphur dioxide, and nitrous acid are negligible.
Denuder/filter pack systems are cost effective because they allow simultaneous sampling of gases and particles using the same pumping system, which can be the most expensive part of the sampling unit. Use of a denuder is also necessary to prohibit reactions between gases and collected particles on the filter media. For example, a significant portion of particulate acidity can be neutralized on the Teflon filter by ammonia. Therefore, a citric acid-coated denuder is used upstream from the filter to remove ammonia from the air sample. An ozone denuder can be used upstream from the Teflon filter to prohibit interactions between ozone and the collected organic particulate matter. The coating of such a denuder is a mixture of nitrite and carbonate salts.
The partition coefficient of different atmospheric species between the particle and gaseous phase can be determined by using denuder/filter pack systems. Sampling of atmospheric semi-volatile compounds remains a challenge and is often inappropriately addressed by atmospheric chemists. For instance, collection of polyaromatic hydrocarbons (PAH) or ammonium nitrate is done using two stage filter packs. In the case of polyaromatic hydrocarbons, the first stage of filter pack is a Teflon filter to collect the particulate PAH, and the second stage is a sorbent (XAD-2 or Tenax) to collect the gas phase PAH. However, the sorbent will collect PAHs which were originally sampled by the Teflon filter, and were later volatilized. With an appropriately coated denuder, the amount of the semi-volatile PAH originally in the gas phase can be determined.
Sampling of ammonium nitrate can also be a problem. Filter pack techniques use a Teflon filter to collect particles and a nylon filter (or a sodium carbonate-coated glass fiber filter) to collect nitrates (in nitric acid form) downstream from the Teflon filter. However, the nitrate on the second filter can originate from vapor phase nitric acid or from the volatilization of ammonium nitrate collected on the Teflon filter. Therefore, it is difficult or impossible to determine ammonium nitrate concentrations unless a nitric acid denuder is used upstream from the filter pack. In addition, the volatilization of nitrate from the Teflon filter can be associated with two different processes: (a) the dissociation of ammonium nitrate: EQU NH.sub.4 NO.sub.3 (p).rarw..fwdarw.NH.sub.3 (g)+HNO.sub.3 (g),
and (b) the neutralization of acid aerosol sulfate by ammonium nitrate: ##STR1##
The relative importance of these two mechanisms can be determined using a denuder/filter pack system (Koutrakis, et al., Submitted for publication to Atmospheric Environment (1992)). Denuders are used to remove nitric acid and ammonia from the air sample. The filter pack contains three filters: a Teflon filter to collect fine particles, a sodium carbonate-coated glass fiber filter to collect nitrate (which can originate from both ammonium nitrate dissociation and acid sulfate neutralization reaction, as shown in the above reactions), and a citric acid-coated glass fiber filter to collect ammonium (which originates form the dissociation of ammonium nitrate). By subtracting the number of ammonium moles collected on the third filter from the number of moles of nitrate collected on the second filter, it is possible to calculate the amount of lost particulate acidity from the Teflon filter.
The annular denuder/filter pack technology has made possible a number of field and laboratory studies which have enhanced understanding of the physical chemistry of inorganic atmospheric pollutants. However, this technology is relatively expensive (approximately $1500 per sampling unit) and time-consuming to use. In fact, coating and extraction of denuders requires skilled laboratory technicians and is labor intensive. The denuder preparation, extraction, and analysis costs are between $200 and $300. Also, shipping of the large units is expensive.
Koutrakis, et al., 26A Atmospheric Environment 987 (1992) (which is not admitted to be prior art to the present application) describe a filter pack for use with annular denuder technology. The annular denuder system used is the Harvard system in which a first denuder is coated with sodium bicarbonate and glycerol, and a second denuder is coated with citric acid and glycerol.
Koutrakis, et al., 22 Environmental Science and Technology 1463 (1988) describe an annular denuder/filter pack system for collection of acidic aerosols and gases. The sampler contains a glass impactor in combination with an annular denuder and filter pack system. The impactor plate is a porous glass disc impregnated with mineral oil. Two annular denuders are provided in series, the first of which is coated with sodium carbonate and glycerol to collect SO.sub.2, HNO.sub.3, and HNO.sub.2. The second denuder is coated in the same way to measure any artifact nitrate and nitrite for correction of the HNO.sub.3 and HNO.sub.2 concentration.
Koutrakis, et al., 22 Atmospheric Environment 157 (1988) describe a method for measuring acidity of atmospheric aerosols by use of an ammonia diffusion denuder. A Harvard impactor was used to collect aerosol particles with aerodynamic diameter below 2.5 microns.
Koutrakis, et al., 12 Aerosol Science and Technology 607 (1990) describe a glass impactor for an annular denuder filter pack system in which a removable porous glass disc is impregnated with mineral oil. Stevens, et al. U.S. Pat. Nos. 4,902,318 and 4,961,966 describe an inlet apparatus for gas aerosol sampling having an elutriator column and an impactor section. The column is provided with a coating of a polytetrafluoroethylene-containing polymer.
Hering, "Air Sampling Instruments" (1989), describes a single stage impactor with ten circular jets in which the impactor surface is an oil-coated porous stainless steel plate.